In practice, the isolated switch-mode power supplies are used in many applications. In the prior art, generally, an isolated switch-mode power supply includes a main power transistor, a transformer, and a freewheeling diode or a freewheeling Metal-Oxide-Semiconductor (MOS) transistor. The primary-side control circuit controls the conduction time of the main power transistor. If the MOS transistor is used in freewheeling, the secondary-side control circuit will control the conduction time of the MOS transistor. In some cases, the isolated switch-mode power supply is required to operate in continuous conduction mode (CCM). In this mode, the freewheeling MOS transistor is turned off and the main power transistor is turned on when the current of the secondary-side winding has not yet reduced to zero. However, if the freewheeling MOS transistor is not turned off on time, the situation where the main power transistor has been turned on while current still exists in the secondary-side winding will be caused. In this case, an excessive high current will flow through the main power transistor, thereby causing a shoot-through phenomenon in the main power transistor, which has a great impact on system reliability and Electro Magnetic Interference (EMI), and can easily damage the power transistor.
FIG. 1 is a partial structural schematic view of a flyback switch-mode power supply in the prior art. In the FIG. 1, M1 is the main power transistor, M2 is the freewheeling MOS transistor, Ci and Co are energy-storage capacitors, L1 and L2 are primary-side winding and secondary-side winding of the transformer, respectively, and N1 and N2 are the number of turns of the primary-side winding and the secondary-side winding, respectively. The primary-side control circuit is connected to the control terminal of M1 to control the on-off state of the main power transistor M1. The secondary-side control circuit is connected to the control terminal of M2 to control the on-off state of the freewheeling MOS transistor M2. When the switch-mode power supply works in the continuous conduction mode (CCM), there is a situation where the main power transistor M1 is turned on while the freewheeling MOS transistor M2 has not yet been turned off. In this case, there is a current flowing through the secondary side winding, and the voltage drop of the main power transistor M1 is: UM1=Ui+(N1/N2)*Uo. As a result, a large current overshoot will occur in the main power transistor M1, thereby causing a shoot-through like phenomenon in the main power transistor, which has a great impact on system reliability and Electro Magnetic Interference (EMI).